Controllers such as programmable logic controllers (PLC) have been implemented in duplex or back-up system configurations where downtime of a system cannot be tolerated. Such a control system delivers high reliability through redundancy. Generally, the duplex configuration incorporates a pair of PLCs assembled in a hot or active standby, or back-up, configuration wherein one PLC is operating in a primary mode and the other PLC is functioning in a secondary or standby/backup mode. The primary controller runs an application by scanning a user program to control and monitor a remote IO network. The other, secondary, controller acts as the active standby controller. The standby controller does not run the application and it does not operate the remote IO. The standby controller is updated by the primary controller with each scan. The standby controller is then ready to assume control of the control system within one scan if the primary controller fails to operate or is removed from operation.
The primary and secondary controllers are interchangeable and can be swapped or switched when desired. Either controller can be placed in the primary state. The active standby configuration requires the non-primary controller to be placed in the standby mode to secure the system's redundancy. The controllers continuously communicate with each other to ensure the operability of the control system. The communication among the controllers is used to determine if a swap of the controllers should be initiated due to a system failure or by election of an operator.
Factory automation systems are increasingly being integrated with communication networks. Control systems are being implemented on networks for remote monitoring and control of devices, processes, etc. System failures involving the primary mode controller that can shut down the control system are avoided by having a back-up controller readily available in hot/active standby mode to replace the failing primary mode controller.
Signal communication between device modules on a network requires network identifiers, i.e., Internet Protocol address, Media Access Control address; to be assigned to these devices throughout the network. Problems arise when the network device or controller fails and must be replaced. The failing primary controller on a network cannot be readily exchanged with a standby controller because the network identifier assigned to the network device, specifically the standby controller, is not readily associated with the primary controller.
Other drawbacks exist due to the physical structure of the control system. The control system utilizes a backplane for operably connecting modules for communicating throughout the network. Generally, data transfers are sent from a central processing unit (CPU) of the primary controller to a network interface module of the primary controller via the primary controller's backplane. A 10 Mb/sec fiber optic cable communicably links the pair of controllers wherein data flows between the network interface module of the primary controller and the network interface module of the secondary controller. From the secondary controller's network interface module, the data must again travel through a backplane to reach the CPU of the secondary controller. Communication between selected network modules involving the backplane hampers the transfer of data and adversely affects the performance of the data transfer. Due to the relatively slow backplane interface, this configuration is inadequate when faster data transfer rates on the control system are desired.
The present invention is provided to solve these and other problems.